Many wire bonding apparatuses employ ball bonding of forming a wire tail extending from a tip of a capillary as a bonding tool into a free air ball by a spark, and bonding the free air ball onto an electrode of a semiconductor device or a substrate using the tip of the capillary.
Gold wires conventionally used to be widely employed in wire bonding, but bonding using copper wires that are less expensive and superior in electric properties have become increasingly more common. However, unlike gold wires, copper wire are susceptible to oxidation, and resulting in an oxidation film formed over a surface of a free air ball when forming the ball by a spark. The oxidation film often causes deterioration in attachment between the ball and an electrode, and thus in poor bonding. Therefore, there is proposed a method of suppressing oxidation of a surface of a free air ball when bonding is performed using copper wires by, for example, forming the ball in an inert gas such as a nitrogen gas or an argon gas (see PTL 1, for example).
However, when blowing an inert gas in one direction to an area in which a free air ball is formed as in a case of a bonding apparatus described in PTL 1, there are problems that decentering may occur in the formed free air ball due to a flow of the inert gas, and that oxidation may occur due to air coming into the area in which the free air ball is formed. Accordingly, there is proposed a method of providing a porous component around a cavity in which the free air ball is formed, decreasing the speed of the inert gas with fine halls in the porous component, and causing the inert gas to be spread evenly within the cavity (see PTL 2, for example).
In contrast, if the temperature of the surface of the free air ball decreases when bonding, there are problems such that bond strength between the free air ball and an electrode may deteriorate, and that irregularities occur in the shape of the free air ball (not rounded, for example). Accordingly, there is proposed a method of ensuring bonding strength by causing a heated reducing gas to flow around a free air ball before, during, and after forming the free air ball to keep the temperature of the free air ball high (see PTL 3, for example). Further, there is proposed a method of performing bonding while suppressing oxidation of a surface of a free air ball and maintaining the temperature of the free air ball high by forming the free air ball in a state in which a heated inert gas flows (see PTL 4, for example).
However, when a structure in which a heated inert gas is sprayed from a gas nozzle is employed as in the conventional techniques described in PTLs 3 and 4, it is necessary to increase a flow rate of the inert gas in order to maintain an atmosphere of the inert gas. Accordingly, there are problems that a large heater for heating an inert gas becomes necessary, and this adversely increases the size of a bonding apparatus, or results in slow operation, making it difficult to perform bonding at high speed.